Heat-developable light-sensitive materials are known in the art. For example, conventional heat-developable light-sensitive materials and heat-development processes are described in Shashinkogaku no Kiso, "Edition of Higin-en Shashin", pages 242 to 255 (Corona Co., Ltd., 1982).
Many different processes for forming color image utilizing heat development have been proposed. Processes for forming color images by the reaction of an oxidation product of a developing agent with a coupler, are described, for example, in U.S. Pat. Nos. 3,531,286, 3,761,270 and 4,021,240, Belgian Patent 802,519 and Research Disclosure, pages 31 to 32 (September, 1975).
However, since the above-described heat-developable light-sensitive materials used for obtaining color image are non-fixing type, silver halide undesirably remains in the light-sensitive material after image formation. The remaining silver halide causes a severe problem in that coloration gradually occurs in white background areas when exposed to strong light or stored for a long period of time. Further, the above-described color image forming processes generally require a relatively long period of time for development. Additionally, the color images formed are unsatisfactory due to a high level of fog and a low image density.
In order to solve these problems, processes have been proposed wherein diffusible dyes are imagewise formed or released upon heating, and the diffusible dyes are transferred to an image receiving material containing a mordant with a solvent such as water, as described, for example, in U.S. Pat. Nos. 4,500,626, 4,483,914, 4,503,137 and 4,559,290 and JP-A-59-165054 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, these processes require a relatively high temperature for heat development. In addition, stability of the light-sensitive materials during preservation is still insufficient. Thus, for the purpose of accelerating development, lowering temperature of development and making processing simple, processes have been proposed in which heat development and transfer of dye are performed in the presence of a base or base precursor and a slight amount of water, as disclosed, for example, in JP-A-59-218443, JP-A-61-238056, 61-238056, and European Patent 210,660A2.
With respect to processes for obtaining positive color images utilizing heat development, many methods have been proposed. For instance, in U.S. Pat. No. 4,559,290, a method is described wherein an oxidized compound, which in its oxidized state does not have a dye releasing ability and is obtained by converting a so-called DRR compound, is coexistent with a reducing agent (including a precursor thereof), the reducing agent is oxidized upon heat development in correspondence to an exposure amount of silver halide, and the oxidized compound obtained from the DRR compound is reduced with the remaining reducing agent which has been not oxidized, whereby a diffusible dye is released. Further, in EP-A-220746 and Kokai Giho 87-6199 (Vol. 12, No. 22), a heat-developable color light-sensitive material containing, as a compound capable of releasing a diffusible dye in a similar mechanism to that disclosed in U.S. Pat. No. 4,559,290, a compound is described which can release a diffusible dye upon reductive cleavage of an N-X bond (wherein X represents an oxygen atom, a nitrogen atom or a sulfur atom).
In the above-described heat-developable color light-sensitive materials, dye providing compounds are included which react under a relatively high pH condition during development processing and generate a dye necessary for forming an image. However, if the dye generating reaction inadvertently occurs during storage or handling of the light-sensitive material, stain increases in a white background area of the image, and thus discrimination of image is diminished. It has been found that the increase in stain is particularly apt to occur when a basic metal salt compound coexists with the dye-providing compound in a layer of the light-sensitive material. The main reason for this phenomenum is attributed to an increase in pH of the layer of the light-sensitive material during preservation.
It has also been found that various other related problems may occur due to the increase in pH of the layer of the light-sensitive material in addition to the above described dye generating reaction. Specifically, organic compounds incorporated into the layer are oxidized by air to form stain.